There are many tutorials for reusable sandwhich bags on the internet. Just do a quick search and you can find on that works for you. Below is what works for me.
Start with one piece of cotton and a coordinating piece of rip stop nylon the same size. My sizes usually range between 5-7 inches wide 11-14 inches long. You can see above the owls have a seam in the middle to keep the images upright on each side. Rip stop is moisture resistent. It will not hold water like a ziploc but can be used for sandwhiches and dry foods.
With right sides together, sew the shorter ends together with a 1/4 inch seam.
Flip the fabric so right sides are out. Topstitch an 1/8th inch from the top of both seams.
Cut velcro 1 inch shorter than the width of the sandwhich bag and sew in place 1/4 inch from the top edge on each side of the bag.
With right sides together, fold the bag in half and serge the side seams. If you don't have a serger you can zigzag the seam or create a french seam for a more finished look.
Turn inside out and admire your work.
You can embelish with with ricrac or other accessories.
We have to label any food that goes into the refrigerator at the office. So I chose to put my name on one of the bags.